Next generation nonvolatile memories that are being focused on as new information storage media include a ferroelectric random access memory (FeRAM), a magnetoresistive random access memory (MRAM), a resistive random access memory (ReRAM), a phase-change memory (PRAM), and the like. These memories have their own advantages, and research and development thereon have been actively progressing in a direction suitable for their use.
An MRAM, among these memories, is a memory element using a quantum mechanical effect called magnetoresistance, is an element which is capable of storing nonvolatile data, with features of high density and high responsiveness with low power consumption, and is a large-capacity memory element that can replace a dynamic random access memory (DRAM) which is a currently widely used memory element.
As a magnetoresistive effect, two effects such as giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) are known.
An element using a GMR effect stores information by using a phenomenon in which a resistance of a conductor located between two ferromagnetic layers is changed according to spin directions of the upper and lower ferromagnetic layers. However, since a magnetoresistance (MR) ratio of a GMR element, which indicates a rate of change of a magnetoresistance value, is as low as about 10%, a reading signal of stored information is small. Therefore, securing a read margin is the greatest challenge in realizing the MRAM.
Meanwhile, as a representative element using a TMR effect, a magnetic tunnel junction (MTJ) element using a change of magnetoresistance according to an MTJ effect is known.
The MTJ element has a laminated structure of a ferromagnetic layer/an insulating layer/a ferromagnetic layer. In the MTJ element, when spin directions of upper and lower ferromagnetic layers are the same, tunneling probability between the two ferromagnetic layers with a tunneling insulating layer interposed therebetween is maximized, and thus a resistance value is minimized. On the other hand, when the spin directions thereof are opposite, tunneling probability therebetween is minimized, and thus a resistance value is maximized.
In order to realize these two spin states, a magnetization direction of either one of the ferromagnetic layers (magnetic material films) is set to be fixed and not to be influenced by external magnetization. Generally, a ferromagnetic layer having a fixed magnetization direction is referred to as a fixed layer or a pinned layer.
A magnetization direction of the other ferromagnetic layer (the other magnetic material film) may be the same as or opposite a magnetization direction of a fixed layer according to a direction of an applied magnetic field. The ferromagnetic layer in this case is generally referred to as a free layer, and serves to store information.
Currently, MTJ elements having an MR ratio more than 50% as a rate of change of resistance are obtained, and are becoming the mainstream of MRAM development.
Meanwhile, an MTJ element using a vertical magnetic anisotropic material among these MTJ elements has entered the spotlight.
Specifically, research on application of an MTJ element using a vertical magnetic anisotropic material to a spin-transfer torque magnetic random access memory (STT-MRAM) or the like has been actively progressing.
A STT-type recording method refers to a method of inducing magnetization reversal by directly injecting a current into an MTJ rather than applying an external magnetic field. The STT-type recording method is advantageous for high integration because there is no need for a separate external conducting wire.
In an MTJ element using vertical magnetic anisotropy, a material used as a pinning layer has an artificial antiferromagnetic material structure. The structure conventionally has an L1/Ru/L1 structure in which Ru is inserted between ferromagnetic layers such as CoPd, CoPt, [Co/Pd], or [Co/Pt].
Currently, in order to apply an STT-MRAM element, it is necessary to finally bond a selection element such as a transistor. A process temperature of such a selection element is about 400° C., and the temperature has a bad influence on the above-described artificial antiferromagnetic material structure.
According to reports so far, Pd or Pt included in a vertical magnetic anisotropic material used to form artificial antiferromagnetic bonding at a temperature of 400° C. to 450° C. is very rapidly diffused during a high-temperature heat treatment process, and thus overall characteristics of the element are degraded.
This diffusion of Pd or Pt not only includes diffusion into an artificial antiferromagnetic layer but also diffusion toward a direction of a seed layer and a capping layer which are used therein. Diffusion into the seed layer and the capping layer has a potential to worsen an interfacial state with CoFeB/MgO/CoFeB bonding.
Therefore, in a structure including the artificial antiferromagnetic material structure, there is a need to develop an MTJ structure having vertical magnetic anisotropy with thermal stability at high temperature.